1. Field of the Invention
This invention relates to flexographic ink compositions. More specifically, the invention relates to a flexographic ink which contains hydrolyzed gelatin as a binder resin and which has particular utility as a preprint ink for paper packaging.
2. Description of the Prior Art
A flexographic printing ink will generally contain a pigment and a binder resin dispersed in water. The binder serves as a carrier for the pigment and affixes the pigment to the surface to be printed. A wide variety of binder systems have been used for flexographic ink compositions, including acrylic and methacrylic polymers and copolymers, rosin modified phenolic resins, polystyrene resins and soy protein.
Flexographic printing inks may be used as preprint inks for corrugated paper packaging materials. The ink is printed upon one side of a paper liner, which is then glued to a series of flutes and then glued to a second paper liner. The preprint ink must be able to withstand the heat and pressure of corrugation, and should preferably display acceptable printing properties such as mileage, strength, wet trap, foam resistance, drying, gloss, plate transfer and water resistance.
xe2x80x9cGelatinxe2x80x9d is a mixture of proteins obtained by hydrolysis of collagen by boiling skin, ligaments, tendons, etc. from various sources such as fish, pig and cow. Its production differs from that of animal glue in that the raw materials are selected, cleaned and treated with special care so that the product is cleaner and purer than glue.
It is known to add hydrolyzed gelatin in minor amounts to ink compositions generally. U.S. Pat. No. 4,770,706 discloses the enzymatic hydrolysis of gelatin for use as a dispersion stabilizer in aqueous inks. Japanese Published Patent Application No. 60-49405 discloses gelatin as one of several possible viscosity control agents in a ball point ink composition containing a fluorescent dye.
Hungarian Patent Publication 47626 discloses a flexographic binder composition which contains 2-4.5 percent by weight xe2x80x9chydrolyzed casein or hydrolyzed gluexe2x80x9d in combination with polyacrylic or polyvinyl polymers having carboxylic and nitrile groups, and a partial alkali metallic and/or ammonia salt of a water-soluble acrylic polymer or vinyl polymer.
Japanese Patent Publication No. 4-325577 discloses a black pigment aqueous ink composition which can include gelatine as a viscosity controlling agent.
European Patent Publication No. 523567 discloses a flexographic, water dilutable bronze or effect printing ink composition which contains 30-45 weight percent of bronze, aluminum and/or a pearlescent pigment and 55-70 weight percent of a second component which includes 80-100% of a vehicle system and 0-20 weight percent of a wax. The vehicle system contains from 60 to 100 weight percent styrene and/or an acrylate dispersion, 0 to 15 weight percent cellulose and/or collagen and/or polyethylene glycol, and 0 to 15 weight percent of one or more glycols.
An object of the present invention is to provide a flexographic ink composition having a superior combination of desirable properties in comparison to conventional flexographic ink compositions.
A feature of the present invention is the presence of at least 15 weight percent hydrolyzed gelatin in an otherwise conventional flexographic ink composition.
An advantage of the present invention is that it can provide a flexographic ink composition which exhibits better heat resistance, mileage, strength, wet trap, foam resistance, drying, stability, and plate transfer than conventional flexographic inks which contain soy as a binder.
The present invention relates to a flexographic preprint ink composition, comprising:
a. at least 3 weight percent of a pigment;
b. at least 15 weight percent of a hydrolyzed gelatin; and
c. water.
The inventor has unexpectedly discovered that it is possible to replace conventional pigment binders (soy and water-soluble polymers such as acrylic and methacrylic polymers and copolymers) with hydrolyzed gelatin in an otherwise conventional flexographic preprint ink composition.
Any pigment suitable for use in a conventional flexographic preprint ink composition is suitable for use in the composition of the present invention. Illustrative pigments include diarylide yellow, orange, napthol red, lithol rubine, phthalocyanine green, phthalocyanine blue, barium lithol, calcium lithol, rhodamine, violet, alkali blue, titanium dioxide and carbon black. Preferred pigments include diarylide yellow such as AAOT yellow (Yellow 14), green shade phthalocyanine blue (Blue 15:13), diansidine orange (PO 16), carbon black (Black 7), rubine shade napthol red (Red 238) and lithol rubine (Red 57).
The pigment may be premilled to reduce its particle size to improve its dispersibility and color strength. Milling can be performed in conventional apparatus using conventional techniques well known to those of ordinary skill in the art.
The pigment may be included in an amount of at least 3 percent by weight of the entire ink composition, preferably from 8 to 25 weight percent. An aqueous pigment dispersion is preferably employed for ease of handling.
The ink composition of the present invention should contain at least 15 weight percent hydrolyzed gelatin, preferably 15 to 40 weight percent, based on the total weight of the composition. Hydrolyzed gelatin may be obtained from cow, fish, bone and pig sources, and is available as an aqueous dispersion. The hydrolyzed gelatin preferably has an average molecular weight of from 1500 to 2500, and is obtained from the cow.
The ink composition of the present invention should have a pH of from 9.0 to 10.0, preferably 9.5 to 9.9. A basic compound, such as an amine, may be added to adjust the pH of the composition. Monoethanolamine is a preferred amine.
The ink composition of the present invention may also contain conventional additives, such as surfactants and defoaming agents, typically found in flexographic ink compositions.
The ink composition of the present invention can also contain a glycol such as propylene glycol preferably in an amount of from 6.5 to 13 weight percent and most preferably from 7.4 to 11.05 weight percent. The propylene glycol can be added to adjust the drying speed of the ink composition.
The viscosity of the ink composition, as measured in a No. 3 Signature Zahn cup, should be at least 11 seconds, and preferably ranges from 11 to 25 seconds.
The flexographic ink composition of the present invention may be prepared by first adding an aqueous pigment dispersion to a mixer. Additional water is added after mixing has begun. Hydolyzed gelatin, and any optional additives such as a surfactant, defoamer, glycol and/or toner, are subsequently added to the mixer. Mixing speed should be controlled to avoid entraining air into the composition. Mixing is continued until an appropriate viscosity and pH are achieved. A typical mixing time is 5 minutes. If necessary or desired, additional water may be added to reduce the viscosity of the composition to a print viscosity of from 11 to 25 seconds, as measured in a No. 3 Signature Zahn cup.
The flexographic ink composition of the present invention may be used as a preprint ink for corrugated paper packaging materials using conventional apparatus and techniques well known to those of ordinary skill in the art. The ink may be printed upon one side of a paper liner, which may then be glued to a series of flutes and then glued to a second paper liner during the corrugation process.
An aqueous overprint varnish composition may be applied over the entire surface of the printed paper liner prior to corrugation to optimize its coefficient of friction for optimum processability within and after leaving the corrugator. The overprint varnish can also ensure the flexographic ink possesses sufficient water resistance. A typical overprint varnish composition includes a polymeric binder, a pH control agent, a defoaming agent, a wax and water.